The long-term objective of the proposed research is to develop a quantitative basis for the engineering of liposome based drug delivery vehicles. Liposomes have significant potential as delivery vehicles for diagnostic and therapeutic agents. For many applications, the efficacy of the liposome as a delivery vehicle depends on its ability to fuse with a target membrane compartment. Thus the ability to control and engineer the fusogenicity of the liposome bilayer is critical to the design and fabrication of liposome based delivery vehicles. However, current understanding of the mechanisms of fusion is inadequate for this purpose. We propose to address this problem by using an integrated and interdisciplinary systems approach that includes modeling, high performance simulation and experimental measurements. We have assembled a group that includes the expertise necessary to address this difficult and complex problem. Simulations of fusion will be performed on highly parallel computers using coarse-grained (mesoscopic) models for lipid bilayers in order to treat the necessary large systems and long times, while retaining the essential chemical detail of the system. Parameterization of the coarse-grained models of lipids will be determined by matching measured quantities such as bending moduli and by coarse-graining structural and energetic quantities calculated in all atom molecular dynamics simulations. Fusion will be simulated starting from hypothetical intermediates, as well as using a weak steering force to motivate the process. Results from these simulations will reveal molecular detail of fusion dynamics, which will be used to design lipids that have predictable fusogenic properties. These lipids will then be tested in liposome based content mixing assays, such as the Tb/DPA system. For example, preliminary simulations suggest that constraining the rotation of C(2) to C(3) bond in the glycerol backbone of a glycerophospholipid, by introducing a bulky group at the C(2) or C(3) position, will reduce the ability of the lipid to participate in fusion. Similarly, a double bond or ring structure between the C(2) and C(3) positions would also reduce fusogenicity. Results from experiments will be used to inform further simulations, producing a process that is interactive between experiment and simulation. Results from the project will lay the foundation for significant additional research using even more sophisticated simulations, and additional experimental methods. In the end, the result of this effort will provide new mechanistic insight into bilayer fusion and ultimately allow the design and engineering of liposomes with predictable fusogenic properties. That in turn will significantly advance the utility of liposomes as drug delivery vehicles, and contribute important insights to biological processes where fusion is involved. [unreadable] [unreadable] [unreadable]